The present invention relates to overlapping joints for gas-tight packing containers of flexible material and to methods for the manufacture of overlapping joints on packing containers of flexible material.
Packing containers of the non-returnable type for liquid contents, e.g. milk, juice, carbonated refreshing beverages or beer are frequently manufactured from laminated, flexible material. The laminated material usually comprises a substantially centrally situated carrier layer of relatively thick cardboard or foamed plastic which is covered on either side by thin, liquid-tight layers of thermoplastic material e.g. polyethylene. The polyethylene layers serve not only as liquid-tight layers but also make possible the heat-sealing of the material. The layers included in the material may vary in respect of type as well as number and thickness, and the material comprises e.g. frequently also a layer of high gas-tightness, e.g. aluminium foil.
When the packing material is to be converted to packing containers, the material is usually subjected to a doubling or reshaping to tubular form so that the opposite edges of the material are made to overlap one another. Subsequently the edges are sealed to one another through heating to such a temperature that the thermoplastic layers in contact with one another melt, whereupon the edge regions are compressed with simultaneous cooling so that a liquid-tight joint is produced. This technique is well-known and has been considered up to now to provide satisfactory joints, but in types of packages where demands made on gas-tightness are very high it has been found that cracks which endanger the gas-tightness may occur, for example when the flexible packing container is handled so that the joints are subjected to stresses. Since even very small cracks, so-called micro-leakage, can imperil the performance of the gas-tight material layer, it is desirable to provide a type of joint in which the risk of micro-leakage has been eliminated.
It is an object of the present invention to provide a joint on gas-tight packing containers which is not subject to the aforementioned disadvantages.
It is a further object of the present invention to provide a joint of the type mentioned in the introduction which can be produced by conventional thermosealing without extensive modifications of technique.
It is a further object of the present invention to provide a joint for flexible material which in spite of deformation of the material retains its tightness and eliminates any risk of micro-leakage.
These and other objects have been achieved in accordance with the invention in that a joint of the type mentioned in the introduction has been given the characteristic that the sealing zone is situated at a distance from the edge of the material facing towards the inside of the packing container. By avoiding any sealing up to the said material edge, the formation of an accumulation of molten thermoplastics along the material edge is avoided, which means that the material retains its flexibility so that bucklings and cracks are avoided on deformation of the sealed material.
The joint in accordance with the invention is combined preferably with a doubling of the edge region of the inner material which has proved to reduce the risk of edge absorption of contents into the material layers at the same time as the risk of leakage on deformation of the joint is not increased.
It is a further object of the present invention to provide a method for the manufacture of a joint on packing containers of flexible material which can be carried out simply and rapidly with packing material and packing machines of a known type.
These and other objects have been achieved in accordance with the invention in that a method of the type mentioned in the introduction has been given the characteristic that the edge regions are compressed at the same time as a bonding agent present between the regions is activated within a portion of this compressed region until a sealing together has taken place.
The method in accordance with the invention makes it possible rapidly and simply to provide a joint in accordance with the invention which is produced in a substantially conventional manner by thermoplastic sealing and without appreciable modifications of the sealing methods customarily used.
It is a further object of the present invention to provide an arrangement for the realization of the abovementioned method by means of which a joint in accordance with the invention can be produced.
These and other objects have been achieved in accordance with the invention in that an arrangement of the type described in the introduction has been given the characteristic that the one sealing jaw includes a region heating the material which is surrounded on either side by regions not heating the material.
With the help of the arrangement in accordance with the invention the heating and sealing of the overlapping edge regions of the material is made possible within a limited sealing zone which is situated at a distance from the edge of the material facing towards the inside of the packing container. The nonheating regions of the sealing jaw ensure that the width of the sealing zone remains limited at the same time as satisfactory cooling of the material is obtained.